Pulse oximeters have become one of the most useful clinical monitors in the modern practice of anaesthesia in providing a convenient estimation of arterial blood oxygen saturation.
Blood absorbs light both in the red(R) and infrared(IR) range. The relative amounts of specific R and IR wavelengths absorbed by the blood depend on the hemoglobin oxygen saturation (SaO.sub.2) of the blood. This principle is used by the pulse oximeter to estimate SaO.sub.2.
Typically, in operation a sensor containing light sources is attached to a patient's finger or ear lobe and the absorbence of the radiation passing through the tissue is measured. The pulse oximeter solves an equation that involves the pulsatile component of the differential absorption of specific red, typically 660nm wavelength and infrared, typically, 930nm wavelength, by transilluminated tissue.
For general background see Anesthesiology, 1983, 59, No. 4, Oct. 349-352, Evaluation of Pulse Oximetry, Yelderman et at; Anaesthesia, 1988, 43, 229-232, The Accuracy Of Pulse Oximeters, Taylor et al; Biomedical Technology Today, Nov./Dec. 1988, 210-214, Asses Anesth. Analg 1989, 68, 368-76, Principals of Pulse Oximetry: Theoritical and Practical Considerations, Alexander et al; D. Tobler,Discussion IX in Payne J. P., Severing Haus J. W. (Editors) "Pulse Oximetry" N.Y. Springer Verlag 1986, p. 185.93.
Thus the pulse oximeter has become a vital tool of non-invasive patient monitoring in enabling the oxygen saturation level of blood to be measured.
Important clinical decisions are frequently made based entirely on blood oxygen saturation measurements obtained by pulse oximetry. However, due to the fact that the pulse oximeter requires the presence of pulsatile blood for its operation it is difficult to quickly, inexpensively and conveniently assess the functioning and accuracy of the oximeter. It is also unfortunate that pulse oximeters cannot be easily calibrated by the user since it relies on built-in calibration curves for its accuracy.
Mendelson et al., IEEE Transactions on Biomedical Engineering, 1989; 36, No. 6, 36,-27, An In Vitro Tissue Model for Evaluating the Effect of Carboxyhemoglobin Concentration on Pulse Oximetry, describes an in vitro method for calibrating a pulse oximeter, which method is rather cumbersome, expensive and requires large quantities of blood. Munley et al, The Lancet, May 13, 1989, 1048-49, A Jest Object For Assessing Pulse Oximeters, describe a test object consisting essentially of a dummy "finger" with a rotating cone. However, this suffers from disadvantages overcome by the present invention. The device of Munley et al requires a test object which is difficult, time consuming and expensive to construct. It requires two hands of an operator and must be perfectly positioned relative to the sensor for accurate operation. The test object cannot be used to estimate the oxygen saturation level of blood nor to test the potential errors introduced by various physiological variables, such as hemoglobin concentration or skin pigmentation, in determining accuracy of pulse oximeters.
In contrast, the test device of the present invention is easy, fast and inexpensive to construct. The sensor need not be perfectly positioned on the test device for an accurate determination. Further, the latter is operated using only one hand of an operator and can be used to estimate the oxygen saturation of blood and to readily estimate the effect of physiological variables on the accuracy of pulse oximeters. Yet further, the amplitude of the signal produced by the device of the present application is controllable by the operator, whereas that of the Munley et al device is dependent on the amplitude of the pulsatile signal produced and fixed by the geometry of the Munley device, which may be too high or too low for some pulse oximeters.
It is an object of the present invention to provide a method of testing the accuracy of pulse oximeters which is quick, inexpensive and convenient.
It is a further object of the present invention to provide a device for use with a pulse oximeter to easily check said oximeter.